Catalytic cracking is a petroleum refining process which is applied commercially on a very large scale. A majority of the refinery gasoline blending pool in the United States is produce by this process. In the catalytic cracking process heavy hydrocarbon fractions are converted into lighter products by reactions taking place at elevated temperature in the presence of a catalyst, with the majority of the conversion or cracking occurring in the vapor phase. The feedstock is thereby converted into gasoline, distillate and other liquid cracking products as well as lighter gaseous cracking product.
During catalytic cracking, heavy material, known as coke, is deposited onto the catalyst. This reduces its catalytic activity and regeneration is desired. After removal of hydrocarbons from the spent cracking catalyst, regeneration is accomplished by burning off the coke which restores the catalyst activity. The three characteristic steps of the catalytic cracking can be therefore be distinguished: a cracking step in which the hydrocarbons are converted into lighter products, a stripping step to remove hydrocarbons adsorbed on the catalyst and a regeneration step to burn off coke from the catalyst. The regenerated catalyst is then reused in the cracking step. For modern refineries, the Fluid Catalytic Cracking Unit (FCCU) produces 40 to 60+% of the gasoline in the gasoline pool. In addition, the FCCU produces a blendstock component for diesel manufacture. Air quality regulations for these transportation fuels will require a further improvement in air quality as mandated by the Clean Air Act. For the FCCU process, there are two routes a refiner can utilize to further reduce the impurity content of these transportation fuels. The first route is via a hydrotreatment process on the feedstock to the FCCU. This hydrotreatment process can by operational severity and design, remove a substantial amount of the feed impurity to produce a gasoline impurity content of 100 ppmw or less. The second route a refiner can take involves the use of a specialized catalyst or additive in the FCCU circulating catalyst inventory that can catalytically remove impurities from the FCCU product distributions. Refiners may elect to use this route for both non-hydrotreated and/or hydrotreated FCCU feedstock derived from various crude sources. In addition, if a refiner utilizes the first route for desired gasoline content, when the hydrotreater is taken out of service for an outage, this specialized catalyst or additive can be utilized to minimize the increase of gasoline impurities during the outage period.
FCC dry gas product contains inert gases (O2, CO, CO2 and N2) that are entrained with the regenerated catalyst as catalyst flows from the regenerator to the reactor. The inerts are contained in the interstices of the catalyst and are required to provide the fluidization medium to allow the catalyst to behave like a fluid. The inerts are typically 0.8 lb/Mlb CCR. They pass through the reactor and main column where they are compressed and go through the gas concentration unit. The inerts reduce the WGC capacity and increase vapor traffic through the gas concentration distillation towers. This constrains unit capacity and reduces LPG recovery. The resulting dry gas product also contains a lower heat value.
Other objects and advantages of the present invention will become apparent to those skilled in the art upon a review of the following detailed description of the preferred embodiments and the accompanying drawings.